1. Field of the Invention
This invention relates to a polymer blend, specifically, to a reactor blend of polypropylenes having different microstructures, i.e., a reactor blend of isotactic polypropylene and syndiotactic polypropylene and a process for making such a blend having increased molecular weight distribution, improved clarity, higher melt flow and higher xylene solubles.
2. Description of the Prior Art
It is known that two or more homogeneous catalysts, such as those based on metallocene compounds, may be combined to effect properties, such as molecular weight distribution. U.S. Pat. No. 4,530,914 discloses use of a catalyst system comprising two or more metallocenes in the polymerization of xcex1-olefins, primarily ethylene, to obtain a broad molecular weight distribution. The metallocenes each have different propagation and termination rate constants. The metallocenes are mixed with an alumoxane to form the catalyst system.
It is known that blends of isotactic and syndiotactic polypropylene can be formed with a Ziegler polymerization catalyst as described in Belgian Patent 538,782. As disclosed in U.S. Pat. No. 3,268,627, a mixture of from about 50-97% by weight of isotactic polypropylene and from about 3-50% by weight of a syndiotactic polypropylene can be formed with up to about 20% by weight of atactic polypropylene.
It is also known to produce polymer blends by polymerizing two or more polymerizable materials in two or more reactors arranged in series. As disclosed in U.S. Pat. No. 4,937,299, a polymerizate is produced in a first reactor and then the first polymerizate is passed to a second reactor wherein a second polymerizate is produced thereby obtaining a blend of the first and second polymerizates.
It is also known to produce a reactor blend of polyolefins in a single reactor by use of a homogeneous catalyst system comprising at least two different metallocenes and an alumoxane. As disclosed in U.S. Pat. No. 4,937,299, two different metallocenes can polymerize ethylene and propylene to form an ethylene-propylene copolymer in a single reactor.
It would be advantageous to provide a process to produce a polymer blend of isotactic polypropylene and syndiotactic polypropylene having improved properties over both homopolymers. Furthermore, it would be advantageous to provide a polymer blend which could be produced in a single reactor.
Accordingly, this invention provides a reactor blend of isotactic polypropylene and syndiotactic polypropylene.
And, an object of this invention is to provide a polymer blend of isotactic polypropylene and syndiotactic polypropylene in a single reactor.
Also, an object of this invention is to provide a polymer blend having up to 4% by weight of syndiotactic polypropylene to produce increased molecular weight distribution.
Further, an object of this invention is to provide a polymer blend having up to 2.34% by weight of syndiotactic polypropylene to improve the clarity or haze of biaxially oriented film.
Additionally, an object of this invention is to provide a polymer blend having from about 2.34% by weight of syndiotactic polypropylene to about 13.9 by weight and a melt flow higher than that for either a syndiotactic homopolymer or an isotactic homopolymer.
As well, an object of this invention is to provide a polymer blend having from about 2% by weight of syndiotactic polypropylene to about 13.9% by weight and a level of xylene solubles that is higher than that for either a syndiotactic homopolymer or an isotactic homopolymer.
These and other objects are accomplished by use of at least one homogeneous catalyst and at least one heterogeneous catalyst, preferably in a single reactor.